As with many populist slogans, "Black lives matter," is one that does not hold up to more than thirty seconds of actual thought.
Whether a life (or, implicitly a death) matters is wholly a matter of individual factors. A 40-year-old brain surgeon who is killed in an auto accident matters, regardless of race, ethnicity, religion, gender or physical attractiveness. On the other side of the spectrum is the death of a career criminal, who has victimized both individuals and society at large for a large part of its life. That life matters, in the sense that the loss of it is a net positive for society. It is generally conceded that the life of a child matters because of its potential to benefit others. But as the child grows up and starts to give hints of what is to come...the scale starts to tip in one direction or another.
How often have "we" read of the death of a Black Yoot to gang violence, who was constantly in trouble with the law, and his mother cries into the camera, saying that her son had just found Jesus. Gimme a break. That life, to be honest, didn't matter much. OTOH we also hear about young Black folks who were doing well in school, in the community, maybe in a sport or two, and were gunned down because they just happened to be in the wrong place and time. Surely that life matters a lot.
"All lives matter" is just as vacuous, and is not helpful. Obviously, not all lives matter; it just depends.